The invention is directed to a web gap control for a corrugator. More particularly, the invention is directed to a method of and apparatus for controlling the size of gap between the trailing and leading portions of a web of corrugated paperboard which has been severed to initiate a new production run.
Conventionally, when changing from an old production run to a new production run, the web is severed at a station between the end of a double facer machine and a slitter-scorer machine. The leading portion of the web proceeds at an accelerating speed through the slitter-scorer machine to a cut-off machine which cuts the leading portion of the web into box blanks for the old production run. These box blanks are directed onto a stacker conveyor. The trailing portion of the web follows. Box blanks of the new production run are cut from the trailing portion of the web and are added to those already on the stacker conveyor. If the gap which has formed between the leading and trailing portions of the web becomes too large, the leading edge of the first box blank of the new production run can bump the already imbricated sheets of the old production run on the stacker conveyor, generally disarranging the box blanks of the old production run.
Various techniques exist for the detection of a web gap in a sheet processing machine. For example, U.S. Pat. Nos. 3,565,423 and 3,507,489 disclose sheet delivery devices wherein a gap in a shingled production run is automatically detected and closed. In U.S. Pat. No. 3,565,423, the gap is created by the deliberate removal of a defective sheet from the run. The gap is optically detected, and a delivery conveyor is stopped momentarily to permit the gap to close.
In U.S. Pat. No. 3,507,489, the gap is also optically detected, and the conveyor is stopped momentarily to permit the gap to close.
It is also known in the paperboard industry to momentarily stop the stacker conveyor until the first sheet or blank of the new run reaches the last blank of the old run on the stacker conveyor. By this technique, the gap between the blanks is closed at the stacker conveyor before the stacker conveyor is re-started. In this case, the first blank of the new run shingles on the last blank of the old run and consecutive blanks of the new run accumulate on the first blank until the stacker conveyor is re-started. The first new run blank may not present sufficient tail to permit retention by the conveyor suction cups or tail grabbers so that the old run can be separated and cleared from the new run on the stacker conveyor.
Heretofore, the problem of preventing bumping of consecutive blanks on the stacker conveyor while insuring retention of the new run blank has not been resolved.
Accordingly, there is need for a control which limits the size of gap between the trailing edge of the last sheet or blank of an old production run and the leading edge of the first sheet or blank of a new production run so that the first sheet falls upon the sheets already shingled on the stacker conveyor at such an angle that the shingled sheets are not disturbed. There must also be provision of sufficient tail on the new sheet for the conveyor retarding means to seize the new sheet to allow the old sheets to be taken away with their arrangement intact.